Assembly Not Found exception during BinaryFormatter.Deserialize

[Fruber Malcome]

I'm getting a very weird exception and hoping someone may be able to help.

I have an Add-In that lives in a .dll (for email reference ai.dll)
ai.dll makes calls into the core part of the application implementation in
another .dll (let's call that app.dll).
app.dll makes a function call into another dll (let's call that one
dep.dll).

dep.dll has some code in it where it deserializes a memorystream using a
binaryformatter.
I step through the code in this function and it properly - opens the binary
file, puts it into a BinaryReader, then I get the byte[] by calling the
br.ReadBytes()., I then create a new memory stream (ms) using that byte[] in
the constructor, I create a new binaryformatter with the default
constructor, I then say:
object obj = bf.Deserialize( ms );

And it's on this line of code that I get: "Cannot find the assembly
dep.dll"??

I look at the stack trace and it's in the dep.dll assembly (which is where I
stepped this far in the code).

The Stack trace is below:
at
System.Runtime.Serialization.Formatters.Binary.BinaryAssemblyInfo.GetAssembly()
at
System.Runtime.Serialization.Formatters.Binary.ObjectReader.GetType(BinaryAssemblyInfo
assemblyInfo, String name)
at System.Runtime.Serialization.Formatters.Binary.ObjectMap..ctor(String
objectName, String[] memberNames, BinaryTypeEnum[] binaryTypeEnumA, Object[]
typeInformationA, Int32[] memberAssemIds, ObjectReader objectReader, Int32
objectId, BinaryAssemblyInfo assemblyInfo, SizedArray
assemIdToAssemblyTable)
at System.Runtime.Serialization.Formatters.Binary.ObjectMap.Create(String
name, String[] memberNames, BinaryTypeEnum[] binaryTypeEnumA, Object[]
typeInformationA, Int32[] memberAssemIds, ObjectReader objectReader, Int32
objectId, BinaryAssemblyInfo assemblyInfo, SizedArray
assemIdToAssemblyTable)
at
System.Runtime.Serialization.Formatters.Binary.__BinaryParser.ReadObjectWithMapTyped(BinaryObjectWithMapTyped
record)
at
System.Runtime.Serialization.Formatters.Binary.__BinaryParser.ReadObjectWithMapTyped(BinaryHeaderEnum
binaryHeaderEnum)
at System.Runtime.Serialization.Formatters.Binary.__BinaryParser.Run()
at
System.Runtime.Serialization.Formatters.Binary.ObjectReader.Deserialize(HeaderHandler
handler, __BinaryParser serParser, Boolean fCheck, IMethodCallMessage
methodCallMessage)\r\n at
System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Deserialize(Stream
serializationStream, HeaderHandler handler, Boolean fCheck,
IMethodCallMessage methodCallMessage)
at
System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Deserialize(Stream
serializationStream, HeaderHandler handler)
at
System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Deserialize(Stream
serializationStream)
at that line of code mentioned above

Any help would be appreciated.
thanks